15's are beautifully functional... tires can be a bit less than larger rims, which means you can drive harder for less... heh...
In all seriousness, 15's are beautiful if your car is a daily driver 'cause the ride is a bit smoother, even with a drop... 16's on the other hand are a nice compromise between looks, weight, & how it affects acceleration... but I guess that's just my opinion...
you really can't go wrong with 15's or 16's, just make sure that they are of a decent weight as not to slow the car down for the sake of looking good...